Analysis of granule breakage in a rotary mixing drum: Experimental study and distinct element analysis

Rotary drums are commonly used in particulate solid industries for mixing, coating and reactions. The process is often accompanied by undesirable breakage of granules. For this reason, a scaled-down version is sometimes used as an attrition testing device. In this work, the attrition of granules inside a rotary drum at 18, 35 and 52 rpm drum rotation speeds for 4000 cycles is studied. The granules used in this study have been produced by extrusion and spheronisation with a size range of 500 to 1000 μm. The rotary drum has an internal diameter of 0.39 m, axial length of 0.3 m and a single baffle. The extent of breakage is quantified by sieving out fine debris which is two sieve sizes smaller than the feed particles. To relate the extent of breakage in the drum to granule characteristics, single granule impact tests have been performed on one type of granule at several velocities. The effects of particle size and impact velocity are analysed and a power-law relationship is fitted between impact velocity and single granule breakage. This information is then used to simulate granule breakage in a rotary drum by Distinct Element Method (DEM). The drum is simulated for 5 rotations at the rotational speeds stated above and the breakage rate is extrapolated to 4000 cycles where it is compared to experimental results obtained. The trends for particle breakage in both experiments (determined by sieving) and extrapolated DEM simulations are in agreement however the orders of magnitudes are different. The comparison shows that the extent of breakage obtained from extrapolated simulations is overestimated at drum speed of 35 and 52 rpm and underestimated at 18 rpm. There is close agreement between experiments and extrapolated DEM simulations for particle breakage at 18 rpm only after 4000. Furthermore, the effect of air drag on the attrition of granules by impact at a drum rotation speed of 52 rpm is investigated, where it is found to significantly reduce the breakage results.     

Very High Volume Aerosol Sampling with a Novel Drum Centrifuge

For chemical analysis of trace compounds, comparatively large amounts of dust have to be collected. If good time resolution is required, very high sampling flow rates are mandatory. The operating principle of the drum centrifuge built to cope with these requirements is based on particle deposition on the inner surface of a porous rotating drum. Due to the rotation, a pressure gradient draws the aerosol into the bore of the axis and from there radially outward through a number of holes into the drum. The aerosol then moves to the periphery of the double-walled drum, which consists of two 0.15-mm-thick metal sheets with 1-mm spacing. Each of these metal sheets is perforated by several rows of small slits resulting in porosity of 16%. The slits in the inner and outer sheet are displaced, so that the particles will be strongly deflected on their way out of the rotating drum. Under the combined action of centrifugal forces and strong streamline deflection in the displaced slits of the two thin-walled drums, the particles are deposited. Flow rate as a function of rpm and collection efficiency as a function of particle size were determined experimentally. For simplicity, only the flow field of two (nonrotating) displaced slits was mathematically analyzed. The resulting 2-D solution of the Navier-Stokes equation was used for deterministic limiting trajectory calculations in the case of large particles. Diffusional motion of small particles was allowed for by Monte Carlo trajectory calculation. The calculated deposition efficiencies agree satisfactorily with the experimental results. At 3000 rpm a flow rate of 1200 m³/hr and efficiencies of 91% for 2.1-μm particles, 75% for 0.6-μm particles, and 48% for 0.04-μm particles were obtained. For easy extraction of the collected particulate matter, the device is equipped with an ultrasonic cleaning bath.     

Axial dispersion of granular material in horizontal rotating cylinders

The discrete element method is used to calculate axial dispersion coefficients for approximately monosized particles in a rotating horizontal cylinder. Axial dispersion within the cylinder is shown to follow Fick's second law, in that the mean squared deviation of axial position of a pulse of particles is proportional to time. The axial dispersion coefficient is found to depend on the particle size, gravity and drum rotation speed, allowing a dimensionless group to be formed using these four quantities. For sufficiently large cylinders, the axial dispersion coefficient is found to be independent of drum diameter. A general argument is given which suggests that axial dispersion in physical beds of approximately monosized particles should follow Fick's second law.     

Microdynamic analysis of particle flow in a horizontal rotating drum

The flow of particles in a horizontal rotating drum is studied based on the results generated by Distinct Element Method (DEM). The simulation conditions are comparable to those measured by means of Positron Emission Particle Tracking (PEPT), with a drum being 100 mm in diameter, 35% filled by spheres of 3 mm diameter, and rotating at a speed from 10 to 65 rpm. The simulation method is validated from its good agreement with the PEPT measurement in terms of the dynamic angle of repose and spatial velocity fields. The dependence of flow behaviour on rotation speed is then analysed based on the DEM results, aiming to establish the spatial and statistical distributions of microdynamic variables related to flow structure such as porosity and coordination number, and force structure such as particle interaction forces, relative collision velocity and collision frequency. An attempt has also been made to explain the effect of rotation speed on agglomeration based on the present findings.     

In-situ monitoring of axial particle mixing in a rotating drum using bulk density measurements

A device is described for measuring changes in the local composition of particulate materials in a rotary mixer by continuously monitoring changes in the bulk density. The bulk density is measured using a small cup mounted to the mixer wall that fills with powder during rotation through the bed of particles in the lower part of the mixer. The mass of material in the cup is measured using a load cell during rotation of the cup above the free surface of the particles, and the cup empties before re-entering the particle bed. For mixing of materials with a difference in either particle density, or packing density, the localised bulk density measurement gives a good measure of mixing progress. The measurement device is demonstrated in a 1 m diameter horizontal rotating drum in which two materials are mixed along the axis of the drum. Measurements of the rate of dispersion along the axis are consistent with other work in inclined rotary kilns and can be fitted with a simple diffusion model for the axial mixing of the two species.     

GAS/PARTICLE AND SIZE DISTRIBUTIONS OF POLYCYCLIC AROMATIC HYDROCARBONS IN AMBIENT AIR IN TOKYO 2001–2002

By using an Andersen sampler equipped with a low-pressure impactor, samples of 12 size-classified (>0.13 μm to <12 μm) airborne particles and samples of gaseous components were taken from the air in Tokyo for continuous periods of 19 weeks in the summer of 2001 and 17 weeks in the winter of 2001–2. The sampling filters were changed weekly. The concentrations of eight polycyclic aromatic hydrocarbons (PAHs) in the particulate and gas-phase samples were measured by reverse-phase high performance liquid chromatography (HPLC) with fluorescence detection. Pyrene was detected in the gas phase in both summer and winter: 59% of the total pyrene detected was present in the gas phase in summer, but this fraction decreased to 40% in winter. In the particle fractions, the summer levels of benzo[k]fluoranthene (BkF), dibenz[a,h]anthracene (dBahA), and benzo[a]anthracene (BaA) peaked in particles of diameter 1.25 μm, and benzo[ghi]perylene (BghiP), benzo[a]pyrene, benzo[b]chrysene (BbC), and dibenzo[a,e]pyrene (dBaeP) peaked in particles of diameter 0.76 μm. In winter, BkF, BghiP, BaA, BbC, and dBaeP levels peaked in particles of diameter 0.52 μm, whereas dBahA peaked in particles of diameter 0.76 μm.     

Mixing of particles in rotary drums: A comparison of discrete element simulations with experimental results and penetration models for thermal processes

The transverse mixing of free flowing particles in horizontal rotating drums without inlets has been simulated by means of the Discrete Element Method (DEM) in two dimensions. In the simulations the drum diameter has been varied from 0.2 to 0.57 m, and the rotational frequency of the drum from 9.1 to 19.1 rpm, for drum loadings of 20% or 30%, and average particle diameters of 2.5 and 3.4 mm. The choice of operating parameters allows for comparison with experimental data from literature. Though simple models for inter-particle interactions have been implemented, the overall agreement is good. The results are presented and discussed in terms of mixing times and mixing numbers that means numbers of revolutions necessary for uniform mixing of the solids. In this way, comparison with penetration models, as typically applied to modelling of thermal processes, is possible. The limitations of such continuum models are pointed out, along with the potential of DEM to replace them, in the long term.     

Aerosol Generation by Modern Flush Toilets

A microbe-contaminated toilet will produce bioaerosols when flushed. We assessed toilet plume aerosol from high efficiency (HET), pressure-assisted high efficiency (PAT), and flushometer (FOM) toilets with similar bowl water and flush volumes. Total and droplet nuclei “bioaerosols” were assessed. Monodisperse 0.25–1.9-μm fluorescent microspheres served as microbe surrogates in separate trials in a mockup 5 m³ water closet (WC). Bowl water seeding was approximately 10¹² particles/mL. Droplet nuclei were sampled onto 0.2-μm pore size mixed cellulose ester filters beginning 15 min after the flush using open-face cassettes mounted on the WC walls. Pre- and postflush bowl water concentrations were measured. Filter particle counts were analyzed via fluorescent microscopy. Bowl headspace droplet count size distributions were bimodal and similar for all toilet types and flush conditions, with 95% of droplets <2 μm diameter and >99% <5 μm. Up to 145,000 droplets were produced per flush, with the high-energy flushometer producing over three times as many as the lower energy PAT and over 12 times as many as the lowest energy HET despite similar flush volumes. The mean numbers of fluorescent droplet nuclei particles aerosolized and remaining airborne also increased with flush energy. Fluorescent droplet nuclei per flush decreased with increasing particle size. These findings suggest two concurrent aerosolization mechanisms—splashing for large droplets and bubble bursting for the fine droplets that form droplet nuclei.

Copyright 2013 American Association for Aerosol Research     

Numerical Modeling of Particle Dynamics in a Cylindrical Chamber Containing a Rotating Disk

Numerical modeling was performed to study the submicron particle dynamics in a confined flow field containing a rotating disk, temperature gradient, and various inlet gas flow rates. The Lagrangian model was employed to compute particle trajectories under the temperature gradient, disk rotation speed, and inlet gas flow rate effects. The trajectories of particles with diameters of 1 μm, 0.1 μm, and 0.01 μm were examined in this study. When the inlet gas temperature was lower than that of the disk, particle-free zones were created due to upward thermophoretic force for 1 μm and 0.1 μm particles. Disk rotation was found to depress the size of the particle-free zone. Particle deposition onto the disk for 0.01 μm particles was possible because of the Brownian motion effect. A detailed evaluation of the particle-free zone size as a function of the temperature gradient, disk rotation speed, and inlet gas flow rate was performed. When the inlet gas temperature was higher than the disk temperature, particle deposition onto the disk was enhanced due to the downward thermophoretic force for 1 μm and 0.1 μm particles. Disk rotation was found to increase the deposition rate. For 0.01 μm particles, Brownian motion was more important than thermophoretic force in controlling particle behavior. The particle deposition rates as a function of the temperature gradient, disk rotation speed, and inlet gas flow rate were performed.     

Bubble–particle collision and attachment probability on fine particles flotation

Particle size is an important parameter in flotation and has been the focus of flotation research for decades. The difficulty in floating fine particles is attributed to the low probability of bubble–particle collision. In this research, the influence of hydrodynamic parameters on collision probability of fine particles was investigated. Collision probability was obtained using Stokes, intermediate I and intermediate II and potential equations. Maximum collision probability was 5.65% obtained with impeller speed of 1100 rpm, air flow rate of 30 l/h and particle size of 50 μm. Also, attachment probability under Stokes flow, turbulent and potential flow conditions was calculated 100, 99.49 and 81.87% respectively. Maximum attachment probability was obtained with impeller speed of 700 rpm, contact angle of 90°, particle size of 20 μm and air flow rate of 15 l/h. Collision angles were obtained between 60.71° and 60.18° and attachment angles were obtained between 9.15° and 59.83°.     

In situ polymerization of uniform poly(urea–formaldehyde) microcapsules containing paraffins under the high-speed agitation without emulsifier

In this paper, uniform spherical poly(urea–formaldehyde) (PUF) microcapsules containing paraffins, which can be used as phase change materials for energy storage, were prepared by in situ polymerization method under high-speed agitation (≥10,000?rpm) without emulsifier. The influence of high-speed agitation on particle size of as-prepared microcapsules and the tightness of microcapsules were also investigated. The results show that, all the microcapsules have <10?μm mean particles-size and narrow-size distribution, and the mean particle size decreases with the increase of agitation rate. Furthermore, when the agitation rate is >16,000?rpm, the effectiveness of reducing particle size by high-speed stirring is not as remarkable as that of lower speed agitation. In order to gain good tightness of PUF microcapsules under the high-speed agitation conditions, the final pH value of reaction solution should be lower down compared with that of conventional agitation. In our investigation, when the agitation rate was 10,000?rpm, microcapsules fabricated at pH value <2.0 were sealed and own good tightness, however, those fabricated at pH value >2.2 were not sealed.     

Effect of gravity in differential mobility analysers. A new method to determine the density and mass of aerosol particles

Mobility analysers select particles according to their electrical mobility. In order to take into account the effect of gravity in these analysers, we introduce the settling velocity in equations governing the behaviour of particles. We deduce a new transfer function taking into account the effect of gravity. Based on these theoretical results, we developed a new application. By simultaneously measuring the relaxation time and the electrical mobility of aerosols, we determine their diameter, their mass and therefore their density. In order to validate this theory, we measure the mass of a latex particle (diameter between 0.1 and 2 μm) with a radial flow differential mobility analyser known under the name SMEC. The agreement between the theoretical and the experimental values is good and the uncertainty less than 5% for latex particles with a diameter greater than 0.5 μm.     

Deposition of Tobacco Smoke Particles in a Low Ventilation Room

Deposition on indoor surfaces is an important removal mechanism for tobacco smoke particles. We report measurements of deposition rates of environmental tobacco smoke particles in a room-size chamber. The deposition rates were determined from the changes in measured concentrations by correcting for the effects of coagulation and ventilation. The airflow turbulent intensity parameter was determined independently by measuring the air velocities in the chamber. Particles with diameters < 0.25 μm coagulate to form larger particles of sizes between 0.25 and 0.5 μm. The effect of coagulation on the particles > 0.5 μm was found to be negligible. Comparison between our measurements and calculations using the theory of Crump and Seinfeld (1981) showed smaller measured deposition rates for particles from 0.1 to 0.3 μm in diameter and greater measured deposition rates for particles larger than 0.6 μm at three mixing intensities. Comparison of Nazaroff and Cass' model (1989a) for natural convection flow showed good agreement with the measurements for particles > 0.1 μm in diameter; however, measured deposition rates exceeded model predictions by a factor of approximately 4 for particles in size range of 0.05–0.1 μm in diameter. These results were used to predict deposition of sidestream smoke particles on interior surfaces. Calculations predict that in 10 hours after smoking one cigarette, 22% of total sidestream particles by mass will deposit on interior surfaces at 0.03 air change per hour (ACH), 6% will deposit at 0.5 ACH, and 3% will deposit at 1 ACH.     

Experimental study of oil particle emission rate and size distribution during milling

Metalworking fluids (MWFs) used in milling generate oil particles through impaction, action of centrifugal forces and evaporation/condensation mechanisms. The oil particles suspended in the factory atmosphere can affect the health of the labor force. In order to study the emission properties of these oil particles, this work investigates the oil particle emission rate and size distribution during milling using an environmental chamber method. Two commonly used operating modes for MWFs were selected, the minimum quantity lubrication (MQL) mode (40?ml/h) and the cooling mode (1 m³/h). The cooling mode without cutting was studied separately for comparison with the cooling mode with cutting. The results show that the oil particle emission rate in milling ranges from 7.2 to 641?mg/h, and the size distribution ranges from 0.265 to 12.5?µm. Evaporation/condensation is the main mechanism in the MQL mode. The majority of oil particles formed by evaporation/condensation are in the range of 0.265 to 1.8?µm. As the tool rotation speed increases, the particle emission rate increases, while the mass mean diameter (MMD) and the sauter mean diameter (SMD) decrease. Oil particles are mainly generated by the action of centrifugal force in the cooling mode, and mainly distributed in the range of 1.8 to 12.5?µm. The particle emission rate increases with the tool rotation speed, and the particle MMD and SMD increase with the tool rotation speed only in the cooling mode without cutting. The particle emission rate ranging from 1.8 to 12.5?µm, as well as PM5 and PM10, are a polynomial function of the square of tool rotation speed during the cooling mode. The coefficient of determination (R²) is above 0.99.

© 2018 American Association for Aerosol Research     

Transverse mixing and heat transfer in horizontal rotary drum reactors

The transverse mixing of quartz sand (mean particle sizes 157, 323, 794 and 1038 μm) and sodium carbonate (soda) (mean particle size 137 μm) has been investigated in a laboratory rotary drum reactor of 300 mm length and 310 mm diam. Solid movement in the drum was observed by means of colored tracers and successive exposures as well as by means of hot tracers and recording the local temperature in the bulk of particles. Three different types of the particles and bulk behaviour could be observed for stickly particles. The time constant of the mixing was estimated as a function of the rotational speed of the drum. The “cooling-down” curves of the bulk of particles were measured in a laboratory oven of 250 mm diam. and 600 mm length. The temperature variation as a function of the time can be described by the Newtonian cooling law, from which the heat transfer coefficient at the wall α_w was estimated.The absolute value of α_w's and their dependence on the contact time and particle diameter cannot be calculated by the heat penetration model, which disregards the film resistance at the bulk/wall contact. By taking into account this resistance a good quality of fitting can be achieved.     

Coal–oil–water multiphase fuel: Rheological behavior and prediction of optimum particle size

As the coal–oil–water slurry is gaining importance in place of fuel oil, a better understanding of handling characteristics is in demand. Therefore, experimental investigations have been carried out to investigate the rheological properties of coal–oil–water suspension containing coal particles of different sizes. Different coal stocks with average particle sizes of 108 μm, 75.7 μm and 62.9 μm have been used. The concentration of solid for the experiment varies from 10% to 50% by weight. All experiments have been carried out in a cup and bob type coaxial cylindrical viscometer. Newtonian, shear thinning and shear thickening behavior of suspension has been observed depending on component content and operating conditions. Study with different particle sizes shows that it is possible to achieve an optimum particle size for better handling of such suspension. A generalized correlation has been developed to predict the apparent viscosity of coal–oil–water suspension incorporating the coal concentration, oil concentration, torque and particle diameter. The experimental data are in well agreement with proposed correlation.     

Experiments on floating bed rotating drums using magnetic particle tracking

Magnetic particle tracking (MPT) was employed to study a rotating drum filled with cork particles, using both air and water as interstitial medium. This noninvasive monitoring technique allows for the tracking of both particle translation and rotation in dry granular and liquid–solid systems. Measurements on the dry and floating bed rotating drum were compared and detailed analysis of the bed shape and velocity profiles was performed. It was found that the change of particle–wall and particle–particle interaction caused by the presence of water significantly affects the bed behavior. The decreased friction leads to slipping of the particles with respect to the wall, rendering the circulation rate largely insensitive to increased drum speed. It was also found that the liquid–particle interaction is determining for the behavior of the flowing layer. The well-defined experiments and in-depth characterization performed in this study provide an excellent validation case for multiphase flow models.     

Development of a High-Pressure Aerodynamic Lens for Focusing Large Particles (4–10 μm) into the Aerosol Time-of-Flight Mass Spectrometer

A new aerodynamic lens system for an online aerosol time-of-flight mass spectrometer (ATOFMS) has been designed and constructed to transmit and allow the analysis of individual particles in the 4–10-μm-size range. Modeling was used to help design the lens within the bounds of ATOFMS instrumental constraints. The aerodynamic lens operates at a high inlet pressure, 3066 Pa (23 Torr), with a unique tapered relaxation region to improve large particle transmission. Every stage of the lens was tested empirically using a combination of particle deposition and light scattering experiments. The critical orifice was found to significantly impact large particle transmission, with orifices <200 μm in diameter completely suppressing large particle transmission. The addition of a virtual impactor allowed for the use of large orifices without any loss of functionality in the ATOFMS. The detection efficiency of the ATOFMS was >10% for particles from 4–10 μm with a peak efficiency of 74 ± 9% for 6-μm particles. With the extended size range provided by this inlet, the ATOFMS can now be extended to investigate single cell metabolomics.

Copyright 2014 American Association for Aerosol Research     

Electrodeposition of nickel/silicon carbide composite coatings on a rotating disc electrode

Composite coatings suitable for protection against wear were prepared by electrodeposition from a nickel Watts solution containing silicon carbide particles maintained in suspension. To obtain a better understanding of hydrodynamic effects on the codeposition process a rotating disc electrode, immersed in a vertical rising flow, was used. The local concentration of embedded SiC along the radius of the disc electrode was studied as a function of suspension concentration, rotation rate and the particle mean diameter. The effect of a rheoactive polymer was also examined. Although it is generally admitted that the particle incorporation rate is governed by a two-step adsorption process, the experimental results show that it is also dependent on the spatial distribution of the wall fluid flow. The normal component of the fluid velocity promotes particle impingement, whereas the parallel component tends to eject the loosely fixed particles. The competition between the forces which tend to maintain particles attached to the surface and the shear force which tends to remove them, depends on several parameters, in particular the surface chemistry and the size of the particles, the flow rate and the current density.List of symbols A, B Tafel coefficients according to metal and particle deposition, respectively - _v volume ratio of the amount of code posited particles (vol %) - _w weight ratio of the amount of codeposited particles (vol %) - C concentration of the particles in suspension in the bath (g dm^–3) - F _adh, F _stagn, F _shear, F _fric forces exerted on particles (adhesion, stagnation, ejection, friction) - overpotential (V) - i current density (A dm^–2) - K Langmuir coefficient (g^–1 dm³) - T constant coefficient in Equation 2 - V _r, V _z radial and normal component of the fluid velocity (cm s^–1) - rotation rate of the disc electrode (rpm) - _c critical rotation rate (rpm)     

Electrochemically instantaneous reduction of conducting polyaniline-coated latex particles dispersed in acidic solution

A cathodic voltammetric wave was observed in an aqueous suspension of mono-dispersed, spherical polyaniline-coated polystyrene particles, whereas no anodic wave was detected. This irreversibility was common to particles with eight different diameters ranging from 0.2 to 7.5 μm. Such irreversibility cannot be found at polyaniline-coated electrodes, and thus is a property of the dispersion of polyaniline latex. The reduction current was controlled by diffusion of dispersed particles. The reduction, being the conversion from the electrical conducting state to the resistive one, should begin at a point of contact between the conducting particle and the electrode in order to be propagated to the whole particle rapidly. In contrast, the oxidation proceeds slowly with the propagation of conducting zone, during which Brownian motion lets the particle detach from the electrode. The number of loaded aniline units per particle, determined by weight analysis, ranged from 6 × 10⁶ (Ø 0.2 μm) to 3 × 10¹¹ (Ø 7.5 μm) and was proportional to 2.9 powers of the particle diameter. The diffusion-controlled current of the cathodic wave was proportional to 2.4 powers of the diameter. The difference in these powers, 0.5, agreed with a theoretical estimation of the diffusion-controlled current, the diffusion coefficient for which was given by the Stokes-Einstein equation.